[Bug gcov-profile/89307] -fprofile-generate binary may be too slow in multithreaded environment due to cache-line conflicts on counters

[marxin at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89307

--- Comment #4 from Martin Liška <marxin at gcc dot gnu.org> ---
I'm just looking at the google/gcc-4.9 branch:
https://android.googlesource.com/toolchain/gcc/+/master/gcc-4.9/

and they have a sampling approach:

/* Transform:

   ORIGINAL CODE

   Into:

   __gcov_sample_counter++;
   if (__gcov_sample_counter >= __gcov_sampling_period)
     {
       __gcov_sample_counter = 0;
       ORIGINAL CODE
     }

which effectively updates edge counters just for a limited time. I would expect
size increase:

Removing basic block 9
Removing basic block 10
main (int argc)
{
  unsigned int PROF_sample.2;
  unsigned int PROF_sample.1;
  long int PROF_edge_counter_6;
  long int PROF_edge_counter_7;
  long int PROF_edge_counter_8;
  long int PROF_edge_counter_9;

  <bb 2>:
  __gcov_indirect_call_profiler_v2 (1005944783, main);
  __gcov_indirect_call_callee = 0B;
  if (argc_2(D) != 0)
    goto <bb 3>;
  else
    goto <bb 6>;

  <bb 3>:
  a = 123;
  PROF_sample.2_13 = __gcov_sample_counter;
  PROF_sample.2_14 = PROF_sample.2_13 + 1;
  __gcov_sample_counter = PROF_sample.2_14;
  PROF_sample.2_15 = __gcov_sampling_period;
  if (PROF_sample.2_14 >= PROF_sample.2_15)
    goto <bb 5>;
  else
    goto <bb 4>;

  <bb 4>:
  goto <bb 8>;

  <bb 5>:
  __gcov_sample_counter = 0;
  PROF_edge_counter_6 = __gcov0.main[0];
  PROF_edge_counter_7 = PROF_edge_counter_6 + 1;
  __gcov0.main[0] = PROF_edge_counter_7;
  goto <bb 8>;

  <bb 6>:
  a = 0;
  PROF_sample.1_10 = __gcov_sample_counter;
  PROF_sample.1_11 = PROF_sample.1_10 + 1;
  __gcov_sample_counter = PROF_sample.1_11;
  PROF_sample.1_12 = __gcov_sampling_period;
  if (PROF_sample.1_11 >= PROF_sample.1_12)
    goto <bb 7>;
  else
    goto <bb 4>;

  <bb 7>:
  __gcov_sample_counter = 0;
  PROF_edge_counter_8 = __gcov0.main[1];
  PROF_edge_counter_9 = PROF_edge_counter_8 + 1;
  __gcov0.main[1] = PROF_edge_counter_9;

  <bb 8>:
  return 0;
}